Negotiating salary when it's already very good

I have a job offer and would like to negotiate it a bit higher, because we should always try to get a little more. However, in my research I'm finding very little that is helping me build an argument. Here's the situation:

• Offer is higher than "Average salary for job X in location Y"
• Offer is higher than Glassdoor
• I have another offer, but in VHCOL area. When scaled using Payscale, this offer is still higher than my other offer.

What can I even say when negotiating? Do I just say "I was hoping more for Z amount"? I should mention this is my first job out of school.

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